The present invention relates to a device for detecting sharp focusing through an optical focusing means such as a photographic lens on a predetermined plane such as a film plane. (The device shall be referred to as "an image focusing device" in this specification for brevity.)
Various types of image focusing systems utilizing photodetectors for sharply focusing a photographic lens at an object have been devised and demonstrated. In the case of single-lens reflex cameras which can interchangeably use various photographic lenses, an image focusing system which detects the state of the image of an object focused through a photographic lens upon a film plane has been widely used. This system can be further divided into various sub-systems. For instance, in one sub-system how sharply the image is being focused is detected; in another sub-system, split image prisms are used so that the sharpness of focus can be detected in terms of the degree of coincidence of the split images; and in a further sub-system, a slit or a micro-lens group is used to detect the sharpness of focus in terms of the phase difference of the focused light rays.
These sub-systems have some common defects. First, it is difficult to detect if a photographic lens is sharply focused at an object if the latter is dark or if the contrast thereof with its background is vague. Furthermore, they use a plurality of photodetectors arranged in a predetermined array so that the fabrication costs are high. Furthermore, they must use complex circuits for processing the output from the photodetectors, which also contributes to an increase in fabrication cost.
There has been devised and demonstrated an image focusing system to solve these problems. In this system, a light beam is emitted from a camera to an object and the light reflected from the object is intercepted to determine if the image of the object is sharply focused or not. However, if this system is applied to the single-lens reflex camera, the relative position between a light-beam emission or projection means and a photodetection means and the control of the light-beam emission or projection means must be varied depending upon the types of interchangeable photographic lenses used. As a consequence, there has not been available a single-lens reflex camera incorporating the image focusing system of the type described.